Code Division Multiple Access (“CDMA”), cdma2000, 3GPP, and W-CDMA mobile communication systems are increasingly deployed (or planned to be deployed) to accommodate increasing usage levels of mobile communication technologies. Within these communication systems, and more particularly within mobile stations (or mobile units, such as CDMA mobile telephones or multimedia devices), a “rake” receiver is currently employed for multipath reception.
A rake receiver typically includes a tapped delay line, a plurality of “rake fingers”, and a conjugate gain or maximal ratio combiner, for this multipath reception. The various rake fingers include correlators to despread a received signal with a selected or assigned Walsh code, and are followed by a conjugate gain or maximal ratio combiner or other filter to generate the desired signal for the user.
Rake receivers have typically been utilized, among other reasons, for their low complexity and robustness to channel measurement error. Rake receivers, however, are not optimal in terms of performance. For example, other types of filter arrangements, such as filters which invert the multipath channel between transmitter and receiver, may excessively enhance additive noise in the channel and are sensitive to and degraded by channel measurement error.
Minimum mean square error (“MMSE”) filtering or other linear feedforward equalization has generally not been employed for CDMA reception. More particularly, MMSE or other linear equalization is typically applied only to communication links having a single-user plus noise in the channel, and in which the signaling waveform is not time varying. In contrast, a CDMA, cdma2000 or W-CDMA channel has multiple users plus noise in the channel; users are separated by codes spanning “N” chips; and the codes (waveforms) change from one symbol to the next due to covering by the long pseudorandom (sector) spreading or scrambling code. For higher-order signaling constellations which require more power, rake receivers are not suitable as they have an error floor and multipath channels due to intra-cell interference.
Implementations of CDMA and of proposed technologies such as cdma2000 and wideband CDMA (W-CDMA or 3GPP) may be optimized, however, with the use of receiver structures other than rake receivers. As a consequence, a need remains for a receiver structure which may be utilized with CDMA technologies, including cdma2000, 3GPP and W-CDMA, which provides improved performance and increased system capacity. In addition, such a receiver should also be compatible with existing communications systems and be capable of cost-effective implementation.